Process for removal of chromium in industrial waste waters



United States Patent 3,371,034 PRUCESS FUR REMOVAL OF CHROMIUM ININDUSTRIAL WASTE WATERS Raymond Richards, 7459 Virginia Park, Detroit,Mich. 48202 Continuation-impart of appiication Ser. No. 560,254, June24, 1966, which is a continuation-impart of application Ser. No.524,495, Feb. 2, 1966. This application Jan. 31, 1967, Ser. No. 622,856

9 Claims. (Cl. 210-51) ABSTRACT OF THE DISCLOSURE A process forchemically removing chromium ions from industrial waste solutions bydirect precipitation with barium carbonate in such solutions acidifiedwith nitric or hydrochloric acid or their salts.

Cross-references to related applications This application is acontinuation in part of copending application Ser. No. 560,254, filedJune 24, 1966, now abandoned, which application was a continuation inpart of application Ser. No. 524,495, filed Feb. 2, 1966, now abandoned.

Background of the invention The present invention relates to a method ofpurifying industrial waste waters containing chromic acid and metallicchromate salts. More particularly, this invention provides a continuousmethod for purifying industrial waste waters, for example, the rinsesolutions of chromium plating operations by removing therefrom thechromium ion, and other heavy metal constituents in such waste waters asan easily separable precipitate, which precipitate containssubstantially all of the chromium ion and other metallic ions that wereinitially present in the waste waters as chromium chromates and othermetallic chromates.

Those skilled in the art of chromium plating have long known that bariumchromate is insoluble in aqueous solutions other than those which areacidic and thus are aware that the chromium ion content of chromiumplating solutions is removable therefrom by precipitating that chromiumion in the form of barium chromate. However, for a long time, it hasalso been known that the direct precipitation of chromate with bariumhas serious objections, including the objection of excessive expense inrelation to the generally used process or" reduction of the chromic ionwhich employs sulfur dioxide, sodium bisulfite, or the like, withsubsequent precipitation with lime; and the objection that the wastesolutions from such direct precipitation contain an unacceptably highconcentration of barium and they cannot be directly discarded intosewers, rivers or streams.

Summary 0 the invention In accordance with this invention, it has nowbeen found that the heretofore known disadvantages of the directprecipitation of chromium ions from waste solutions can be overcome andthat the improved direct precipitation process of this invention iscommercially satisfactory for use either in batch form or continuousprocessing form. The problem of substantially completely removing thechromium ion components from the chromium ion-containing waste solution,while simultaneously avoiding the presence of an undesirable quantity ofbarium in the waste water, is that the simple addition of an excess ofbarium carbonate to such a chromium ioncontaining waste solution isineffective to precipitate all of the chromium ion from that solution asbarium chromate.

It has been found that direct precipitation of the chromium ion contentof such a chromium ion-containing solution can be satisfactorilyeffected with an excess of barium carbonate if the waste solution isfirst acidified with hydrochloric or nitric acid or the barium saltsthereof in an amount sufiicient to convert all of the metallic chromatespresent in that waste solution into the salt of the acid which is added.The addition to such an acidified waste solution of an excess of bariumcarbonate causes the precipitation of all, or substantially all, of thechromium ion content of the solution as barium chromate and also effectsthe precipitation of the metallic ion content of the waste solution asthe hydroxide, or complex hydroxide, and such hydroxide precipitatenormally adsorbs to the surface of the barium chromate and is easilyremoved therewith. The resulting precipitate has the filtrationcharacteristics of a crystalline material and is easily filtered, orotherwise separated :by known means, from the waste solution. When thesematerials are employed, the conditions above stated are followed and themajor portion of the purified waste solution is recirculated, orrecycled, to the chromic acid plating or chromic dip rinse solutiontank, any quantity of waste solution which is discarded contains such asmall concentration of barium that it is directly discardable intosewers, rivers or streams. The process of this invention, therefore,provides a commercially successful direct precipitation method forpurifying chromium ion-containing solutions either on a batch or :acontinuous basis.

The rinse waters in an industrial chromium plating installation, or therinse waters from a chromate dip operation normally contain from about200 to about 1,000 parts per million of the hexavalent and trivalentchromium ion, calculated as CrO but none of these rinse waters containonly the chromium ion as contaminants. In the case of the chromiumplating process rinse waters, the rinse solutions are normallycontaminated with ions of the metals which have been chromium plated inthe solution, such as iron, cadmium, zinc, nickel, copper, etc. In thecase of the rinse waters from a dichromate dip process, the rinse watersnormally contain ions of sodium, iron, zinc, cadmium, nickel, copper,etc. While the dilute rinse solutions ordinarily contain a higherconcentration of chromium ions than any of the other metallic ions, theproportion of such other metallic ions can vary substantially, dependingupon the particular metal being plated or dipped and the particularprocess being empoyed.

In attempting to remove all, or substantially all, of the chromium ionfrom such rinse, or waste solutions it was found that the addition ofbarium carbonate to the rinse or waste solution caused the solution tobecome more basic, or the pH of the solution to rise numerically.Increasing the amount of barium carbonate added to the solution causedthe acidity of the solution to decrease to such a degree thatinsufficient barium carbonate would dissolve in the solution toprecipitate all of the chromium ion present in that solution. Underthese conditions, it was noted that even though a substantial excess ofbarium carbonate over that amount stoichiometrically required to formbarium chromate with all of the chromium ion in the solution was addedto the solution it was not possible to directly precipitate :all thechromium ion from that solution. In such instances, it was ordinarilynoted that the pH of the solution had risen to above about 5.5 andusually was above about 6.0, but the specific pH value varied as afunction of the concentration of chromium ion :and other metallicchromates present in the waste solution and many times reached about6.5.

It was found that all, or substantially all, of the chromium ion contentof such solutions was removable as a barium chromate precipitate, andthe other metallic ion content was concurrently removable with thebarium chromate precipitate as absorbed, or occluded, metallic hydratesor hydroxide if the rinse solution was preliminarily, or concurrentlywith the addition of the excess of barium carbonate, treated wtihhydrochloric or nitric acid or the barium salts thereof; the amount ofhydrochloric or nitric acid, or the barium salt thereof, that isrequired varies from solution to solution that is to be purified. Forany given waste solution, however, the amount of acid that is preferredis the stoichiomctric amount required to convert all of the heavy metalchromatcs which are present in the waste solution to the chloride ornitrate salt of the heavy metal. The acid, or barium salt thereof, canbe added on a batch or continuous basis; in either circumstance anexcess of barium carbonate should be added relative to that quantitystoichiometrically required to prc cipitate all of the chromium ion asbarium chromate, e. g. from l.21.4 times the required amount. Thisaddition of barium carbonate maintains the pH, or acidity, of thesolution such that the heavy metal salts hydrolyze to the metallichydrate or hydroxide as the barium chromate precipitates from thesolution, and these hydrates and hydroxides absorb on or occlude in thebarium chromate precipitate and are removed therewith.

The best method for determining the amount or" acid required for a givenrinse, or waste, solution is to first analyze the waste solution forchromium ion concentration by routine analytical procedures. Second, anamount of barium carbonate from 2 to 4 times the stoichiometric amountrequired to form barium chromate with all of the determined chromium ionin the solution is added and the solution agitated. After agitation andthe precipitate has settled it will be noted that the solution remainsyellow in color; hydrochloric or nitric acid, or the barium salt thereofand preferably nitric acid or barium nitrate is then slowly added to thesolution with agitation until the color disappears. The measured amountof acid or salt added is the correct quantity for use on subsequentcorresponding quantities of that Waste solution, and rou tinecalculation provides the quantity for batchwise or continuousprocessing.

In the process of this invention, it is desirable to avoid the additionof acid in excess of that required to convert the heavy metal chromatesinto the corresponding heavy metal chloride or nitrate since such excesshydrochloric or nitric acid will react with excess barium carbonatepresent to form the corresponding barium salt, i.e., barium chloride orbarium nitrate, and these salts are soluble in the solution from whichthe precipitate is removed. While such barium chloride, barium nitrateor barium bicarbonate materials, which are soluble in the treatedsolutions from which the precipitate is removed by filtration orotherwise, can be easily removed therefrom by conventional precipitationas insoluble barium sulfate by adding sulfuric acid or sulfuric acidsalt thereto, it is preferred to avoid such a secondary purificationstep. When the quantity of acid which is added to the rinse solution iscontrolled, as closely as possible to that quantity which is requiredonly for the conversion of the metallic chro mates in the solution tothe corresponding salt thereof, the secondary purification step isavoided and the overall efficiency of the process is thus increased. Itwas found that in a rinse solution to which only the required quantityof hydrochloric or nitric acid or barium salt thereof had been added, inthe presence of an excess of barium carbonate, after stirring and at theend of the pecipitation, a very small quantity of barium bicarbonateremains in the efiluent after filtration or decantation and the quantityof chromate or chromium ion in a solution is so small as to beundetectable by ordinary analytical methods. The precipitate issubstantially crystalline, precipitates cleanly from the solution and iseasily removed from the waste water by ordinary filtration, settling, ordecantation procedures. The precipitate is primarily barium chromate butalso contains the other metallic ions in the form of heavy metalhydroxides, or complex hydrates of the heavy metals originally presentin the waste solution. Brief description of the drawings The process ofthe invention is illustrated in the single flow sheet which illustratesthe utilization of the process of this invention in treating the Waste,or rinse waters, from a chromium plating solution, or a dichromate dipsolution. As shown on the flow sheet, the plating process or dip processis normally carried out in a chromium plating solution tank, generallydesignated lit), and the parts treated therein are then passed instraight line fiow direction, as indicated by the arrows ll, 13 and i5,first through rinse tank 12, thereafter through second rinse tank 14,and finally through hot water tank 16, from which the chromium plated orchromium coated parts are in a condition ready for further processing orultimate utilization.

The chromium electroplated or dipped parts from tank 10 drag over orcarry into, first rinse tank 12 some oi tne chromium plating or chromiumdip solution from that tank and as parts continue to be rinsed in tank12 the solution builds up with the contaminating chromium ions, inhexavalent and trivalent form; the rinse water in first rinse tank 12will also become contaminated, in time, with the other metallic ionchromates, such as iron, zinc, cadmium, copper, nickel, or the like, asabove generally explained, and the same condition will occur in secondrinse tank 14, with continued processing, but normally to a somewhatlower concentration than builds up in first rinse tank 12. In normalinstallations, second rinse tank 14 is arranged to enable circulation ofa portion of the solution therein back to first rinse tank 12 throughline 13, with fresh water being continuously, or periodically, suppliedto second rinse tank 14, as needed, through line 19. Thus, the rinse, orwaste water from first rinse tank 12, which is removed for processingthrough line 20, to the treatment tank 21, contains any of a number ofwater soluble chromium compounds, including chromic sulfate, chromicacid, potassium, and sodium dichromate, and iron, zinc, copper, andchromium chromates or the like. The rinse or waste solution in treatmenttank 21 is purified by adding to that tank an excess of barium carbonateand the hydrochloric or nitric acid constituent, in the proportionsabove stated, as indicated by the arrows thereon. Treatment tank 21 isequipped with agitation means of conventional type, not shown, and theagitated solution in treatment tank 21 is then fed through line 22 intosettling tank 23 and the precipitate containing the barium chromate andother metallic hydroxides or complex hydrates is allowed to settletherein and is periodically removed therefrom through solids line 25 forsubsequent disposal as a solid waste. The decanted liquid, or solution,from which the precipitate has been separated is returned to secondrinse tank 14 through line 24, and, as above generally described, thissolution contains a minor quantity of barium bicarbonate and smallquantities of the barium salt of the acid used, i.e., barium chloride orbarium nitrate. It is to be understood that in the operation of theprocess, maximum effort should be eX- pended to employ accurateprocedures in controlling the quantity of acidic constituent introducedcontinuously into treatment tank 21, such that any undesired excess ofacidic constituent is a relatively small quantity. Settling tank 23 isalso provided with line 25 for removing solids from the tank, and it isto be understood that the removal of solids through line 25 encompassesthe simple draining of solids therefrom or may include a subsequentfiltration step. The excess effluent from settling tank 23 is removable,preferably after filtering, through line 26 directly to sewer, river orstream, as desired. In either continuous or batch operations the excesscfiluent is removable through line 26 to subsequent treatment equipmentfor precipitation of barium salts as barium sulfate.

It will be understood that ordinary equipment required in carrying outthe batch or continuous process of the present invention such as pumps,tanks, agitators, valves, sludge removal facilities, sumps, and thelike, are necessary to carry out the process, but are omitted from theflow sheet, since they are unnecessary to an understanding of theimportant process steps.

Description of the preferred embodiments The rinse water from the firstrinse tank in a chromium plating commercial installation was analyzedand found to have the following composition.

Waste composition: Oz./gal. CI'Og as H2CI'O4 Cr as Cr (CrO 0.0002 1% asFe (CrO 0.0002 Ni as NiCrO, 0.0001 S0,: 0.00025 For each 1000 gallons ofthe rinse water in a treatment tann, barium carbonate was added to thetank in the amount of 8.75 lbs. Using barium nitrate as the acidicconstituent, 3.376 ozs. of barium nitrate were added to the tank and themixture was vigorously agitated for about 58 minutes. At the end of theagitation, the mixed slurry was transferred to a settling tank andallowed to settle overnight. The precipitate had a bright yellow color.After separating the precipitate in the settling tank the efiiuent wasreturned to the second rinse tank in the chromium plating installationas needed to maintain the level therein. Analysis of thcefliuent showedthe effluent to contain about 12-20 parts per million of barium asbarium bicarbonate and barium nitrate.

EXAMPLE II This example illustrates the process of this invention in itscontinuous form. The waste solution being continuously purified was therinse solution taken from the first rinse tank in a chromium platinginstallation for automotive parts; the rate of rinse solution treatmentwas approximately 1000 gallons per hour, and 950 gallons per hour of thepudified solution were recycled to the second rinse tank in the platingline. Approximately 50 gallons per hour of the treated solution, afterfiltration, were discarded.

The waste solution had the following average composition.

Waste composition: Oz./gal. CrO as H CrO 0.08 Cr as Cr '(CrO 1 0.0004 1%as Fe (CrO 0.0004 Ni as Ni CrO 0.0002 80,: 0.0005

Barium carbonate, in the form of an approximately 30% by weight aqueousslurry, was metered into the treatment tank at the rate of 17.5 lbs/hr.and barium nitrate was fed at the rate of 6.75 ozs./hr. A bright yellowprecipitate was formed and separated from the efliuent in the settlingtank; the effluent, being recycled and discarded, had an averageanalysis of 25 to 40 parts per million barium nitrate and bicarbonate.

The same average composition waste solution when treated by adding 17.5lbs/hr. of barium carbonate and 5.35 ozs./hr. of barium chlorideoperated satisfactorily to remove the chromium ion and produce anefiiuent containing about 20 to about 45 parts per million of barium asbarium chloride and barium bicarbonate.

While it will be apparent that the preferred embodiments of theinvention are well calculated to fulfill the objects above stated, itwill be appreciated that the invention is susceptible to modification,variation and change without departing from the proper scope or fairmeaning of the subjoined claims.

What is claimed is:

1. A method for purifying industrial waste waters containing chromiumions and metallic chromate salts comprising the steps of (1) acidifyingsaid waste waters by adding thereto an acidic constituent selected fromthe group consisting of hydrochloric acid, nitric acid and the bariumsalts thereof in an amount sutficient to convert the said metallicchromate salts in said waste water into the salt of said acidicconstituent added thereto, (2) adding an amount of barium carbonate tosaid solution in excess of that required to stoichiometricallyprecipitate all of the chromium ions from said solution as bariumchromate, (3) removing from said solution the insoluble precipitatecontaining said chromium ions and other metallic ions that wereinitially present in said waste water, and (4) discarding at least aportion of the solution from which said precipitate was separated.

2. A method in accordance with claim 1 wherein at least a portion of thesolution from which said precipitate was separated is recycled.

3. A method in accordance with claim 1 wherein said acid is nitric acid.

4. A method in accordance with claim 1 wherein said acid is hydrochloricacid.

5. A method in accordance with claim 1 wherein said acid is bariumnitrate.

6. A method in accordance with claim 1 wherein said acid is bariumchloride.

7. A continuous method for purifying industrial waste waters containingthe chromium ion which comprises the steps of (1) feeding said wastewaters to a retainer vessel, (2) concurrently adding to said waste waterin said vessel an acidic constituent selected from the group consistingof hydrochloric acid, nitric acid and the barium salts thereof in anamount sufficient to convert the metallic chromates in said waste waterinto the salt of said acidic constituent added thereto and an amount ofbarium carbonate to said solution in excess of that stoichiometricallyrequired to precipitate all of the chromium ion from said solution asbarium chromate, ([3) agitating said solution and removing the insolubleprecipitate from said solution, and (4) recycling at least a part ofsaid liquid from which said precipitate was separated.

S. A process in accordance with claim 7 wherein said acidic constituentis barium nitrate.

9. A process in accordance with claim 7 wherein said acidic constituentis barium chloride.

References Cited UNITED STATES PATENTS 6/1967 Laney 210-49 OTHERREFERENCES MICH E E. ROG R ima y Exami er

